The electrowinning of metals is becoming increasingly important by providing for efficiency of metal recovery and concomitant energy conservation and reduced pollution. There has therefore been an increased interest in the development of a stable, inexpensive, inert anode for the electrowinning of metals from acid solutions. Such a development would provide for a substantial saving in time for the break-in of lead-lead dioxide anodes; reduce the amount of silver or antimony required for the alloying of conventional anodes; and decrease the amount of lead deposited on the cathode as an impurity during the subsequent use of lead containing anodes.
Lead dioxide deposited on a titanium substrate gives an anode that is relatively stable, but does not have the desired life. To increase the life of this anode, it was felt that it might be necessary to have a more dense and uniform coating as well as a substrate surface that was not passive.
The reversal of direct current has been used in an attempt to alter the crystal structure to eliminate dendrite growth and pit formation in zinc, copper, and nickel electrodeposition, in both electrowinning and electrorefining. For example, see
Vene, Y. Y., and S. A. Nikolaeva. (Investigation of the Effect of Periodic Changes in Current Direction in the Electrodeposition of Copper From Sulfate Baths.) Zhurnal Fizicheskoi Khimii, v. 29, No. 5, 1955, pp. 811-817; PA1 Volkov, L. V., and V. N. Andrushenko. (Use of Alternating Current for Improvement of Nickel Electroplating.) Tr. Proektn. NauchnoIssled. Inst. "Gipronikel," v. 62, 1975, pp. 99-104; abs. in Che. Abstracts, v. 84, 1976, No. 142479W. PA1 Mantell, C. L. Industrial Electrochemistry. McGraw-Hill Book Company, Inc., New York, 3rd ed., 1950, p. 80; PA1 Grube, G., and H. Gmelin. (The Influence of Superimposed Alternating Current on Anodic Ferrate Formation.) Z. Elektrochem., v. 26, 1920, pp. 153-161; abs. in Chem Abstracts, v. 14, 1920, p. 2446; PA1 Tucker, S. A., and H. G. Loesch. The Influence of Superimposed Alternating Current on the Electrodeposition of Nickel. J. Ind. Eng. Chem., v. 9, No. 9, 1917, pp. 841-844; PA1 Dzhaparidze, L. N., A. G. Shakarishvili, D. G. Otiashvili, V. P. Pruidze, and R. V. Chagunava. (Preliminary Studies on the Superposition of Alternating Current on Direct Current During Electrolytic Production of Maganese Dioxide.) Pererab. Margantsevykh Polimental. Rud Gruz., 1970, pp. 138-143; abs. in Chem. Abstracts, v. 76, 1972, No. 30087V; PA1 Skirstymonskaya, V. I. (Effect of Superimposed Alternating Current on the Electrodeposition of Zinc and Copper.) J. Applied Chem., v. 10, 1937, pp. 617-622; abs. in Chem. Abstracts, v. 31, 1937, No. 6975; PA1 Izgaruishev, N. A., and N. T. Kudryavtzev. (The Influence of Alternating Current on Current Efficiency in Electrolytic Precipitation of Metals.) Z. Elektrochem., v. 38, 1932, pp. 131-135; abs. in Chem. Abstracts, v. 26, 1932, No. 2924; PA1 Isgarishev, N., and S. Berkman. (The Effect of Alternating Current Upon Polarization in the Electrodeposition of Metals.) Z. Elektrochem., v. 31, 1925, pp. 180-187; abs. in Chem. Abstracts, v. 19, 1925, No. 2168.
However, the use of periodic reversal of direct current cannot give the wide possibilities of crystal structure control that is possible with superimposing alternating current onto direct current.
Superimposing alternating current onto direct current has been shown to keep metal substrates from becoming passive during electrolysis, as discussed in the following references:
The patent literature also contains various types of current applications during electrodeposition. U.S. Pat. No. 2,443,599 involves a method for electroplating metal to a metal substrate using alternating current superimposed on direct current to produce a positive, pulsing voltage. U.S. Pat. No. 2,515,192 also superimposes alternating current on direct current to achieve uniform distribution. U.S. Pat. No. 2,706,170 uses alternating current superimposed on direct current for reducing internal stress. While it is evident that the method of superimposing alternating current onto direct current has been utilized for various purposes, it is clear that these applications do not produce a long-lived electrode since grain structure analysis was heretofore unknown.
Other related techniques are disclosed in U.S. Pat. Nos. 3,720,590 and 4,026,781.